Filtercake removal using exothermic in-situ nitrogen-producing reactants

ABSTRACT

Removal of filtercake particles from a wellbore is achieved by injecting two (2) water-soluble nitrogen-containing salts and an organic acid resulting in the generation of nitrogen gas and elevated temperature.

FIELD OF THE INVENTION

This invention relates generally to a process for the removal offiltercake from oil wells producing medium to heavy oil, and also oilwells found in low draw-down pressure reservoirs. More particularly, thepresent invention relates to a process for a filtercake breaker systemachieved by a theromochemical reaction.

BACKGROUND OF THE INVENTION

In the drilling of oil wells, the drilling fluid is used to aid in thedrilling of boreholes into the earth. The liquid drilling fluids, whichare often referred to as drilling muds, are classified into three maintypes of muds. They are:

-   -   water-based muds, which can be either dispersed or        non-dispersed;    -   non-aqueous muds, usually referred to as oil-based mud; and    -   gaseous drilling fluid which includes a wide range of gaseous        materials.

The drilling fluid serves many roles, including providing a hydrostaticpressure to prevent the fluids in the formation from entering into thewellbore, keeping the drill bit cool and clean during the drillingoperation, the carrying out of drill cuttings and to suspend the drillcuttings when drilling is halted during removal and re-entry of thedrilling assembly. The particular drilling fluid or mud that is employedis chosen carefully for its particular function in to order avoid damageto the reservoir formation, limit corrosion and determine filtrationrate and filtercake properties.

During the drilling operation, reservoir drilling fluid is circulatedwithin the drilling equipment to cool the drill bit, reduce frictionbetween the drill string and the sides of the borehole, and also to forma filtercake to prevent filtrate leak-off into the formation. Thedriving force for the formation of the filtercake is the higher pressureapplied to maintain the stability of the borehole.

The filtercake that is formed serves to restrict the inflow of fluidsinto the wellbore during the drilling process and to set the stage forthe completion of the well. Cake properties such as thickness,toughness, slickness and permeability are important because the cakethat forms on permeable zone in the wellbore, can cause the pipe tostick and other drilling problems. If the filtercake damage createdduring the drilling process is not removed prior to or during completionof the well, reservoir productivity will be compromised.

The prior art has taught a variety of methods for filtercake removal toenhance oil production. In U.S. Pat. No. 6,110,875, which isincorporated herein by reference, xanthan molecules are degraded usingan xanthanase enzyme complex that is stable at temperatures above 250°F., such as those temperatures that are found in some wellbores andprocess streams. The xanthanase enzyme complex is produced by a novelsoil bacterium. The xanthanase enzyme complex may be used to removexanthan based formation damage, such as drilling filtercakes andfiltrates, or to remove xanthan based filtercakes and/or residues whichare present in processing equipment. The xanthanase enzyme complex mayalso be used to reduce the viscosity of xanthan-containing fluids, suchas hydraulic fracturing fluids, blocking gels, drilling muds, andprocess fluids. The xanthanase enzyme complex may also be used inconjunction with other well or process treatments, such as stimulationand cementing operations, to improve the effectiveness of thesetreatments.

For example, U.S. Pat. No. 6,818,594, which is incorporated herein byreference, proposes methods and related compositions for altering thephysical and chemical properties of the substrate used in hydrocarbonexploration, such as in downhole drilling operations. In a preferredembodiment, a method is disclosed which involves formulating a fluidtailored to the specific drilling conditions that contain one or moreinactivated enzymes. Preferably, the enzyme is an activated byencapsulation and pH responsive material. After the fluid has beenintroduced into the well bore, one or more triggering signals, such as achange in pH, is applied to the fluid that will activate or reactivatethe inactivated enzyme, preferably by causing it to be released from theencapsulation material. The reactivated enzyme is capable of selectivelyacting upon a substrate located downhole to bring about the desiredchange in the chemical or physical properties of the substrate.

U.S. Pat. No. 7,712,536, which is incorporated hereing by reference,discloses a treatment fluid and a method for cleaning boreholefiltercake using the treatment fluid, wherein the filtercake containsreservoir drilling fluid solids. The method consists of pumping thetreatment fluid downhole in contact with the filtercake to be removed toestablish a differential pressure between the treatment fluid and theformation adjacent the filtercake, and evenly propagating treatment ofthe filtercake during the differential pressure period so as to delaythe breakthrough by the treatment fluid for a period of from 1 to 12hours. The treatment fluid comprises a fluoride source containing a 1.2to 5 molar fluoride concentration, and another acid or combination ofacids to provide a pH between 1.8 and 5.

In U.S. Pat. No. 7,855,168, which is incorporated herein by reference, amethod and composition for removing filtercake is disclosed. Amultifunctional fluid is disclosed that addresses a new concept in theremoval of filtercake. A composition is disclosed which comprises acarrier fluid, a surfactant, a fluorine source and an organic stabilizerwhich is able to minimize the precipitation of fluorine.

Another example of treating the filtercakes is found in U.S. Pat. No.7,709,421, which is incorporated herein by reference, which discloses asingle phase microemulsion to improve the removal of filter cakes formedduring drilling with oil-based muds. The single phase microemulsionremoves oil and solids from the deposited filter cake. Optionally, anacid capable of solubilizing the filter cake bridging particles may alsobe used with the microemulsion. In one embodiment the acid may be apolyamino carboxylic acid. Skin damage removal from internal andexternal filter cake deposition can be reduced. In another embodiment,the single phase microemulsion may contain a filtration control additivefor delaying the filter cake removal, its destruction or its conversion.

SUMMARY OF THE INVENTION

The invention provides a method and a composition for removingfiltercake from wells producing heavy to medium oil utilizing athermochemical reaction mechanism. An exothermic chemical reaction isinitiated downhole when two (2) added nitrogen-containing water-solublesalts are contacted with an acid to liberate nitrogen gas and heat.

The placement of the reactants includes injecting one nitrogen salt withthe acid through coiled tubing, while injecting the second salt byitself through the production tubing. Upon mixing downhole in the targetzone containing the filtercake, the reaction results in the generationof heat and nitrogen gas which effectively removes the filtercake fromthe formation in the wellbore.

The heat generated reduces the viscosity of the oil, thereby enhancingits mobility and the nitrogen gas generated reduces the hydrostaticpressure of the oil column. The generated heat and the nitrogen willalso improve dissolution of the filtercake by the acid. Both actionsresult in an improved clean-up of the wellbore. This effectiveness ofthe treatment will be more pronounced with heavy oil and also in lowdraw-down reservoirs. This method also has a positive impact on oil wellproductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the apparatus employed in the practice of the process ofthe present invention;

FIG. 2 is a typical filtration curve associated with filtrate invasionand depictions of the filtercake formation;

FIGS. 2A and 2B are, respectively, schematic illustrationsrepresentative of the filtercake before and after treatment inaccordance with the process of the present invention;

FIGS. 3A, 3B and 3C, 3D are, respectively, illustrations representativeof photographs of a laboratory filter element coated with a filtercakebefore and after treatment in accordance with the invention;

FIG. 4 is a temperature profile of the process of the present inventionin the High Pressure/High Temperature Filter Press; and

FIG. 5 is a pressure profile of the process of the present invention inthe High Pressure/High Temperature Filter Press.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a method and composition to remove filtercakedamage from wells producing heavy to medium oil. The method provides theoptimal conditions for filtercake removal treatments. The systemcontains two nitrogen containing salts in addition to an organic acidwhich functions to dissolve the carbonate material present in thefiltercake.

The reaction mechanism of the two nitrogen salts in the presence of theacid is set forth in the following equation:

NH₄Cl(l)+NaNO₂(l)+(H⁺)→N₂(gas)+NaCl+2H₂O ΔH=75 kcal/mole  (1)

The process of the invention thus provides for the simultaneousdissolution of the filtercake and the generation of heat and nitrogengas. The temperature of the reaction can be from about 80° C. to about150° C., with a temperature of 100° C. to about 120° C. being preferred.

Unlike conventional clean-up fluids, the method of the present inventionhas the following advantages:

-   -   a. The treatment is environmentally friendly. The reactants are        water-soluble nitrogen-containing salts and the reaction        products are heat, N₂ and water. The preferred        nitrogen-containing salts are ammonium chloride and sodium        nitrite.    -   b. The generated heat serves to reduce the viscosity of the oil,        thus enhancing oil mobility and the nitrogen gas generated        reduces the effective hydrostatic pressure of the oil column.        Both effects improve reservoir clean-up, especially with heavy        oil and low draw-down pressure wells.    -   c. Both the generated heat and nitrogen gas help to overcome        required draw-down to initiate flow into the well; this is        especially helpful for low draw-down pressure wells. There is no        need for nitrogen lift.    -   d. The generated heat promotes the reaction of the organic acid        with the carbonate material that is present in the filtercake        and the generated gas produces agitation that enhances the acid        filtercake interaction and removal of filtercake from the        wellbore face.    -   e. The process can also include the introduction of one or more        water-based emulsion breakers, viscosity reducers and wetting        surfactants.    -   f. The process is effective with weak organic acids.

The present invention also aids in solving several problems in theremoval of filtercake damage from wells producing heavy to medium oil,including the following:

-   -   g. poor clean-up of filtercake in wells with low draw-down        pressure;    -   h. poor clean-up of filtercake in wells with heavy oils; and    -   i. poor clean-up of filtercake damage in deep wells which        typically have high hydrostatic pressure due to the column of        oil.

Turning to FIG. 1 which is a schematic representation of the method ofthe present invention, wherein an oil bearing zone 10 is depicted havinga well 11 extending into the zone. Production tubing 12 and a coiledtubing 13 extend down into a wellbore 14 which extends into the oilbearing zone 10.

During the drilling operation, a filter cake 15 is formed at the bottomof well casing 16 where the filtercake 15 meets the oil bearingreservoir 10. Shown in the illustrations of FIGS. 2A and 2B is thebuild-up of internal and, then later, the external filtercake which mustbe removed to improve the flow of hydrocarbons from the reservoir rock.

Sodium nitrite and 15 vol. % acetic acid are injected through theproduction tubing 12, while ammonium chloride is injected via a pump 17through the coiled tubing 13. The rate of injection of the sodiumnitrite +15% vol. acetic acid is at about twice the rate of the ammoniumchloride injection. While 15% vol. acetic acid is preferred

10 vol % to 12 vol % can be used to advantage and the acid employed canbe selected from the group consisting of formic acid, propionic acid,lactic acid and benzoic acid.

Upon mixing downhole, both heat and nitrogen gas are generated whichcauses the acid to dissolve the filtercake.

EXAMPLE 1

In order to assess the efficacy of the method of the present inventionon a laboratory scale, a High Pressure (HP)/High Temperature (HT) FluidLoss Test was conducted. The conditions and results are set forth below:

High Pressure/High Temperature Fluid Loss Test

A standard HP/HT filter press was used to perform tests under staticconditions. Several HP/HT fluid loss tests were conducted to build-upthe filter cake under static conditions using lab mud samples. Thegenerated filtercake was then treated in accordance with the process ofthe present invention, Nitrogen/Heat Generation System (N/HGS), alongwith other additives. The test procedure employed is described below:

-   -   1. Saturate the (10 μm) disc in (44 g of NaCl+336.7 ml H₂O) NaCl        completion brine.

2. Load the disc in the fluid loss cell.

3. Fill the cell with 350 ml filtered brine solution.

-   -   4. Place the cell in heating jacket and allow 20 minutes to        reach 160° F. (71° C.).

5. Pressurize the cell to 10 psi; open lower valve and record time for300 ml effluent to pass through disc. Record initial flow rate inproduction direction.

-   -   6. Remove excess fluid and turn the ceramic disc over.    -   7. Repeat steps 3 through 5. Record initial flow rate in        injection direction.    -   8. Bleed pressure, pour off excess brine and fill cell with 300        ml of drill-in fluid (DIF).    -   9. Pressurize cell to overbalance pressure (200 psi) and allow        20 minutes to reach test temperature of 160° F.    -   10. Begin leak-off test and record filtrate volume at 1, 4, 9,        16, 25, 36 and 180 minutes.    -   11. Bleed pressure and siphon out majority of remaining DIF        without disturbing the filter cake.    -   12. Carefully remove the disc and take photograph. Replace the        disc in cell.    -   13. Add clean-up treatment to cell and soak it for 2 hours.    -   14. Bleed pressure and siphon out majority of remaining        treatment fluid.    -   15. Rinse cell with a filtered brine solution.    -   16. Remove the disc and take photograph.    -   17. Place cell in heating jacket and allow 20 minutes to come to        test temperate, 160° F.    -   18. Pressurize cell to 10 psi and record time to flow 300 ml in        the production direction.    -   19. Calculate clean-up efficiency by dividing the time required        to obtain 300 ml of the filtrate after addition of DIF by that        obtained before addition of the DIF.

FIG. 3 provides support that the process of the present invention(N/HGS) is very effective as a filtercake breaker system and theobtained returned permeability of 81% is well justified. As shown inFIG. 3, the process of the present invention, which is a nitrogen andheat generating system (N/HGS), was able to deteriorate and destroy thefiltercake. Also, the inherent properties of N/HGS will help to removedifferent types of mud damage as shown in the series of illustrations ofFIGS. 3A-3D, discussed in more detail below.

Referring now to the representative illustrations of 3A-3D that arebased on before and after photographs, FIGS. 3A and 3B show a circularlaboratory filter disc test element that contains a filtercake on theupper surface of uniform thickness and consistency before beingsubjected to the treatment in accordance with the process of theinvention. Following exposure to the treatment, the filtercake of FIGS.3C, 3D has fissures and has been loosened and removed from the filtermedium in some areas.

EXAMPLE 2 Thermodynamics Determination of N/HGS:

The test procedure used to determine N/HGS thermodynamic was as follows:

-   -   1. Place 64 ml (30 ml of NaNO₂ and 30 ml of NH₄Cl) in a 1000 ml        beaker.    -   2. Add 36 ml SGN Reagent B to the beaker.    -   3. Stir gently using a magnetic stirrer for one (1) minute    -   4. Record temperature    -   5. Add 0.5 ml of acetic acid to the beaker to start the reaction    -   6. Record the temperature at 30 second intervals until reaction        is completed    -   7. Repeat experiment to eliminate errors.

The same testing procedure that was used in Example 1 with the HP/HTfilter press was used to generate the temperature and pressure profilesshown in FIGS. 4 and 5. The results obtained show that significant heatgeneration and nitrogen production which was observed as pressureincreased is associated with the exothermic reaction of the N/HGScomponents.

EXAMPLE 3

This example is representative of the process of the present invention.

Employing the apparatus depicted in FIG. 1, Pill A, whose composition isset forth in Table 1 below, is injected into the coiled tubing and PillB, whose composition is identified in Table 1 below, is injected intothe production tubing. Pill A is injected at twice the rate of Pill B.The term “pill” as used here means batch or mixture.

TABLE 1 General formulation of N/HGS Component Composition ConcentrationPill A NaNO₂ + 15 vol % Acetic Acid 66.67 vol % Pill B NH₄Cl 33.33 vol %

EXAMPLE 4

Employing the apparatus depicted in FIG. 1, Pill A and Pill B,consisting of the ingredients in Table 2 below were injected through theproduction tubing, 12, while Pill C which consists of the ingredientsrecited in Table 2, was injected through the coiled tubing 13.

TABLE 2 Pill Name Composition Formulation Pill A 40.1 19.7 20.1 20.1  51vol % Pill A vol % vol % vol % vol % NaNO₂ Acetic EC-103 EC-106 AcidPill B 100% Diesel 0.6 Vol % Pill B Pill C 50 vol % 50 vol % 48.4 vol %Pill C  NaNO₂ NH₄Cl

As can be seen below in Table 3, the field results obtained from well C,treated in accordance with the method of the present invention, yieldeda normalized oil rate of 1, while well A treated by acid precursor andwell B treated by enzyme and in-situ acid generated results which aresignificantly lower in normalized oil rates.

TABLE 3 Well Treatment System Normalized Oil Rate A 1 0.57 B 2 0.78 CN/HGS 1

Well C shows the highest oil rate following N/HGS treatment. If it isassumed that well C has a flow rate of 1,000 bbls/day, then well B willhave a flow rate of 780 bbls/day and well A will have a flow rate of 570bbls/day.

The invention has been described with reference to specific embodimentsthereof, and provides a method and composition for the removal offiltercake particles from a wellbore in a hydrocarbon reservoir.However, it will be evident to those of ordinary skill in the art thatvarious modifications can be made without departing from the scope ofthe invention as set forth in the appended claims. Accordingly, thespecification is to be considered as being illustrative rather thanrestrictive.

What is claimed is:
 1. A method of removing filtercake particles formedin a wellbore extending into a hydrocarbon reservoir, which comprises:a. injecting at least two water-soluble nitrogen-containing salts and anorganic acid into a wellbore containing a filtercake sufficient toliberate nitrogen gas and generate heat; b. dissolving the filtercake bythe organic acid, the nitrogen gas formed and the heat generated fromthe reaction between the acid and the nitrogen salts in the borehole. 2.The method of claim 1, wherein the water-soluble nitrogen-containingsalts are NH₄Cl and NaNO₂ and the organic acid is acetic acid.
 3. Themethod of claim 2, wherein the NaNO₂ and the acetic acid are injectedseparately from the NH₄Cl.
 4. The method of claim 2, wherein the NaNO₂and the acetic acid are injected through the wellbore's connectivetubing and the NH₄Cl is injected through the wellbore's productiontubing.
 5. The method of claim 2, wherein the NH₄Cl is injected at abouttwo times the rate that the NaNO₂ and acetic acid are injected.
 6. Themethod of claim 2, wherein the heat generated is about 75 kcal/mole.